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Isotope effects in ice Ih: A path-integral simulation

Ice Ih has been studied by path-integral molecular dynamics simulations, using the effective q-TIP4P/F potential model for flexible water. This has allowed us to analyze finite-temperature quantum effects in this solid phase from 25 to 300 K at ambient pressure. Among these effects we find a negative thermal expansion of ice at low temperatures, which does not appear in classical molecular dynamics simulations. The compressibility derived from volume fluctuations gives results in line with experimental data. We have analyzed isotope effects in ice Ih by considering normal, heavy, and tritiated water. In particular, we studied the effect of changing the isotopic mass of hydrogen on the kinetic energy and atomic delocalization in the crystal, as well as on structural properties such as interatomic distances and molar volume. For D$_2$O ice Ih at 100 K we obtained a decrease in molar volume and intramolecular O--H distance of 0.6% and 0.4%, respectively, as compared to H$_2$O ice.